Thermoacoustic tissue scanner

ABSTRACT

A thermoacoustic imaging system including an electromagnetic radiation source for irradiating said tissue to stimulate a thermoacoustic response, a coupling media for acoustically coupling the response to an acoustic sensor array, and an acoustic sensor array. The array comprises sensors arranged on a surface, which is rotatable about an axis to position said sensors in a plurality of positions for detecting the thermoacoustic response. The angular extent of the surface about the axis, subtends an angle that is less than a full revolution, streamlining the device and permitting flexibility in positioning the radiation source and other elements of the device. The source of electromagnetic radiation for irradiating the tissue is a plurality of sources arranged about the tissue and producing synchronized electromagnetic radiation in varying polarizations or phases to irradiate said tissue with electromagnetic radiation of a desired polarization. A thermoacoustic imaging system sized to be held within the human hand is also disclosed.

FIELD OF THE INVENTION

[0001] The present invention relates to imaging properties of tissuebased upon differential absorption of electromagnetic waves in differingtissue types by photo-acoustic techniques.

BACKGROUND OF THE INVENTION

[0002] It is well established that different biologic tissues displaysignificantly different interactions with electromagnetic radiation fromthe visible and infrared into the microwave region of theelectromagnetic spectrum. While researchers have successfully quantifiedthese interactions in vitro, they have met with only limited successwhen attempting to localize sites of optical interactions in vivo.Consequently, in vivo imaging of disease at these energies has notdeveloped into a clinically significant diagnostic tool.

[0003] In the visible and near-infrared regions of the electromagneticspectrum, ubiquitous scattering of light presents the greatest obstacleto imaging. In these regions, scattering coefficients of 10-100 mm⁻¹ areencountered. Consequently, useful numbers of unscattered photons do notpass through more than a few millimeters of tissue, and imagereconstruction must rely on multiply-scattered photons. While effortspersist to use visible and infrared radiation for imaging through thicktissue (thicker than a few centimeters), clinically viable imaginginstrumentation has not been forthcoming.

[0004] In the microwave region (100-3000 MHZ), the situation isdifferent. Scattering is not as important, since the wavelength (inbiologic tissue) at these frequencies is much greater than the “typical”dimension of tissue inhomogeneities (≈1 μm). However, the offsettingeffects of diffraction and absorption have forced the use of longwavelengths, limiting the spatial resolution that can be achieved inbiologic systems. At the low end of the microwave frequency range,tissue penetration is good, but the wavelengths are large. At the highend of this range, where wavelengths are shorter, tissue penetration ispoor. To achieve sufficient energy transmission, microwave wavelengthsof roughly 2-12 cm (in tissue) have been used. However, at such a longwavelength, the spatial resolution that can be achieved is no betterthan roughly ½ the microwave length, or about 1-6 cm.

[0005] In vivo imaging has also been performed using ultrasoundtechniques. In this technique, an acoustic rather than electromagneticwave propagates through the tissue, reflecting from tissue boundaryregions where there are changes in acoustic impedance. Typically, apiezoelectric ceramic chip is electrically pulsed, causing the chip tomechanically oscillate at a frequency of a few megahertz. The vibratingchip is placed in contact with tissue, generating a narrow beam ofacoustic waves in the tissue. Reflections of this wave cause the chip tovibrate, which vibrations are converted to detectable electrical energy,which is recorded.

[0006] The duration in time between the original pulse and itsreflection is roughly proportional to the distance from thepiezoelectric chip to the tissue discontinuity. Furthermore, since theultrasonic energy is emitted in a narrow beam, the recorded echoesidentify features only along a narrow strip in the tissue. Thus, byvarying the direction of the ultrasonic pulse propagation,multi-dimensional images can be assembled a line at a time, each linerepresenting the variation of acoustic properties of tissue along thedirection of propagation of one ultrasonic pulse.

[0007] For most diagnostic applications, ultrasonic techniques canlocalize tissue discontinuities to within about a millimeter. Thus,ultrasound techniques are capable of higher spatial resolution thanmicrowave imaging.

[0008] The photoacoustic effect was first described in 1881 by AlexanderGraham Bell and others, who studied the acoustic signals that wereproduced whenever a gas in an enclosed cell is illuminated with aperiodically modulated light source. When the light source is modulatedat an audio frequency, the periodic heating and cooling of the gassample produced an acoustic signal in the audible range that could bedetected with a microphone. Since that time, the photoacoustic effecthas been studied extensively and used mainly for spectroscopic analysisof gases, liquid and solid samples.

[0009] It was first suggested that photoacoustics, also known asthermoacoustics, could be used to interrogate living tissue in 1981, butno subsequent imaging techniques were developed. The state of prior artof imaging of soft tissues using photoacoustic, or thermoacoustic,interactions is best summarized in Bowen U.S. Pat. No. 4,385,634. Inthis document, Bowen teaches that ultrasonic signals can be induced insoft tissue whenever pulsed radiation is absorbed within the tissue, andthat these ultrasonic signals can be detected by a transducer placedoutside the body. Bowen derives a relationship (Bowen's equation 21)between the pressure signals p(z,t) induced by the photoacousticinteraction and the first time derivative of a heating functions,S(z,t), that represents the local heating produced by radiationabsorption. Bowen teaches that the distance between a site of radiationabsorption within soft tissue is related to the time delay between thetime when the radiation was absorbed and when the acoustic wave wasdetected.

[0010] Bowen discusses producing “images” indicating the composition ofa structure, and detecting pressure signals at multiple locations, butthe geometry and distribution of multiple transducers, the means forcoupling these transducers to the soft tissue, and their geometricalrelationship to the source of radiation, are not described.Additionally, nowhere does Bowen teach how the measured pressure signalsfrom these multiple locations are to be processed in order to form a 2-or 3-dimensional image of the internal structures of the soft tissue.The only examples presented are 1-dimensional in nature, and merelyillustrate the simple relationship between delay time and distance fromtransducer to absorption site.

[0011] The above-referenced U.S. Pat. No. 5,713,356, filed by thepresent inventor, details a diagnostic imaging technique in which pulsesof electromagnetic radiation are used to excite a relatively largevolume of tissue and stimulate acoustic energy. Typically, a largenumber of such pulses (e.g., 100 to 100,000), spaced at a repetitioninterval, are generated to stimulate the tissue. The above-referencedU.S. Pat. No. 5,713,356 discloses methods for measuring the relativetime delays of the acoustic waves generated by a sequence of suchpulses, and for converting these time delays into a diagnostic image.

SUMMARY OF THE INVENTION

[0012] In one aspect, the invention features a thermoacoustic imagingsystem including an electromagnetic radiation source for irradiatingsaid tissue to stimulate a thermoacoustic response, a coupling media foracoustically coupling the response to an acoustic sensor array, and anacoustic sensor array. The array comprises sensors arranged on asurface, which is rotatable about an axis to position said sensors in aplurality of positions for detecting the thermoacoustic response. Theangular extent of the surface about the axis, subtends an angle that isless than a full revolution, so that the surface is substantiallysmaller than the sensor bowl described in the above-referenced U.S. Pat.No. 5,713,356, streamlining the device and permitting greaterflexibility in positioning the radiation source and other elements ofthe device.

[0013] In the described specific embodiment, the array surface is madeof a plurality of flat sections, in an arc, arranged such that ageometric center of each section is equidistant from a common point onthe axis of rotation. Each section each carries a plurality of acousticsensors, the sections nearer to the axis of rotation carrying feweracoustic sensors than the sections further from that axis.

[0014] The acoustic sensors may comprise singe piezoelectric sensors, ordual sensors arranged side-by-side or coaxially with a combiner forcombining their signals to form a combined signal.

[0015] A sensor array such as described may also be used, in conjunctionwith an ultrasound beam steering circuit, as an ultrasonic imagingdevice. Specifically, the beam steering circuit is coupled to theacoustic sensors and stimulates the sensors to produce an ultrasoundbeam directed into said tissue. Echoes of this beam are received by thesensors and combined to form an image of the tissue.

[0016] In a second aspect, the invention features a thermoacousticimaging system, in which the source of electromagnetic radiation forirradiating the tissue is a plurality of sources arranged about thetissue and producing synchronized electromagnetic radiation in varyingpolarizations or phases to irradiate said tissue with electromagneticradiation of a desired polarization.

[0017] In the described specific embodiment, the sources comprisewaveguides positioned to launch electromagnetic radiation toward saidtissue, in varying polarizations or phases, such as vertical andhorizontal polarization and/or relative phase shifts of zero and ninetydegrees.

[0018] In a further aspect, the invention features a handheldthermoacoustic imaging system for imaging structures of tissue.Specifically, thermoacoustic imaging is performed with a device sized tobe held within the human hand, which has a source of electromagneticradiation for irradiating the tissue to stimulate a thermoacousticresponse, and an acoustic sensor array for detecting the thermoacousticresponse.

[0019] The above and other objects and advantages of the presentinvention shall be made apparent from the accompanying drawings and thedescription thereof.

BRIEF DESCRIPTION OF THE DRAWING

[0020] The accompanying drawings, which are incorporated in andconstitute a part of this specification, illustrate embodiments of theinvention and, together with a general description of the inventiongiven above, and the detailed description of the embodiments givenbelow, serve to explain the principles of the invention.

[0021]FIG. 1 is a perspective view of a thermoacoustic computedtomography system;

[0022]FIG. 2 is a view of the sector scanner of the system of FIG. 1;

[0023]FIG. 3 is a diagram of the sensor positions achieved duringrotation of the scanner of FIG. 2;

[0024]FIG. 4A-1 is an illustration of a first embodiment of a dualacoustic sensor, and FIG. 4A-2 is an illustration of a second embodimentof a dual acoustic sensor;

[0025]FIG. 4B is an illustration of the combination of frequencyresponse characteristics of two acoustic sensors;

[0026]FIG. 5 is an illustration of the electronic circuitry used in acombination TACT and ultrasound imaging system using the scanner of FIG.2;

[0027]FIGS. 6A, 6B, 6C, 6D, 6E-1 and 6E-2 are illustrations ofconfigurations of polarization and phase applied to the waveguide arrayof FIG. 1 to achieve alternative polarizations of the radiation in abreast using the system of FIG. 1; and

[0028]FIG. 7 is an illustration of a hand-held TACT scanner.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

[0029]FIG. 1 illustrates the structure of a thermoacoustic computedtomography (TACT) system in accordance with one embodiment of thepresent invention. In the embodiment 10 shown in FIG. 1, a tank 12 isfilled with an acoustic coupling media such as distilled and deionizedwater. Tank 12 has a cylindrical shape and includes in its interior,immersed in the coupling media, a rotary stage 14 supporting anelectromagnetic radiation system as well as an acoustic sensor. Asdescribed in the above-referenced U.S. Pat. No. 5,713,356, theelectromagnetic radiation system is used to stimulate a thermoacousticresponse within tissue, which thermoacoustic response is detected by theacoustic sensor.

[0030] More specifically, the electromagnetic radiation system comprisesan electromagnetic splitter 16 for dividing electromagnetic energy froman external source for delivery to each of eight waveguides 18.Electromagnetic radiation is carried to waveguides 18 by coaxialconductors 20 associate with each waveguide 18, and connecting thewaveguide 18 to splitter 16. An external positioning ring 21 connects tothe lower surface of each waveguide 18 to maintain the relative positionof each waveguide 18 within tank 12. Waveguides 18 are positionedannularly about the central area of tank 12 so as to irradiate a humanbreast 22 positioned in the central area of tank 12. Waveguides 18 arepositioned below the upper surface of tank 12 and angled upwardly towardthe central area of tank 12 so as to produce relatively uniformirradiation of the breast 22.

[0031] Also positioned within tank 12 is a detector array 24 carrying aplurality of piezoelectric or other forms of acoustic sensors fordetecting thermoacoustic signals produced within the tissue of thebreast 22 in response to electromagnetic radiation emitted by thewaveguides 18. The detector array 24 subtends a small angle around theentire circumference of the tank 12. By rotation of the rotary stage 14,waveguides 18 and detector array 24 may be rotated to a plurality ofrotational positions to thereby collect thermoacoustic signals producedin each angular direction from the breast 22 under irradiation ofelectromagnetic radiation from waveguide 18. The resultingthermoacoustic signals collected at a plurality of positions surroundingthe breast 22 can then be used in a reconstruction algorithm such asdescribed in the above-referenced U.S. Pat. No. 5,713,356, to producethermoacoustic images.

[0032] Referring now to FIG. 2, details of the detector array 24 can beexplained. The array is comprised of three flat-faced subarrays 28, 30and 32, each of which subtends an angle of 11.25° relative to thecentral axis of the tank 12. The array is formed of three flat sections28, 30 and 32 to simplify manufacturing, however the array could also beformed of a smoothly curved hemispherical section subtending a similarangular portion about the axis of the cylindrical tank 12. In use, thearray is rotated 11.25° between each data acquisition to each of 32discrete positions about the vertical axis of tank 12, which positionsspan a fill 360° about this axis. The sections 28, 30 and 32 arepositioned such that geometric center of each section is equidistantfrom a common point, as illustrated by line segments 26. The commonpoint is centralized within the breast when immersed within tank 12, andpreferably on the central axis of cylindrical tank 12.

[0033] Referring to FIG. 3, the relative positions of the transducers ofthe transducer array, as the array is rotated in 11.25° increments, canbe appreciated. Nine discrete positions of the array are illustrated inFIG. 3, corresponding to one-quarter revolution, or 90° of rotation ofthe array about the breast 22. It can be appreciated from FIG. 3 thatthe resulting transducer positions are evenly distributed across theresulting hemispherical surface surrounding the breast 22, as isdesirable for reconstruction.

[0034] The detector array illustrated in FIGS. 1-3 has a variety ofpotential advantages over the hemispherical bowl array illustrated inthe above-referenced U.S. Pat. No. 5,713,356. Specifically, the spacingof the acoustic detector locations is more nearly uniform over thesurface of a hemisphere using a detector array such as illustrated inFIGS. 1-3. Also, the detector array is more compact and allowsflexibility of the location and distribution of RF emitting elementsaround the breast, as compared to a hemispherical bowl sensor. Finally,the detector array may be less expensive to manufacture owing to itsless complex mechanical structure as compared to a hemispherical bowl asshown in the above-referenced U.S. Pat. No. 5,713,356.

[0035] Referring now to FIG. 4A-1 and FIG. 4A-2, possible structures forthe transducers within the detector array can be explained. In oneembodiment, the acoustic sensors and the detector array comprise singlepiezoelectric elements chosen for their acoustic properties to match tothe frequency ranges expected to be produced by the thermoacousticeffect used under the present invention. In an alternative embodiment,where a particularly broad acoustic bandwidth is desirable, eachacoustic sensor in the detector array may be comprised of two or morediscrete acoustic sensors, such as two discrete piezoelectric elements,which collectively are used as an acoustic sensor. As illustrated inFIG. 4A-1, a first sensor 34 and a second sensor 36 may be positionedphysically adjacent at each detector sensor site, and the signals fromthese sensors may be delivered to a signal combiner 38 to produce acombined output signal. Alternatively, as illustrated in FIG. 4A-2, afirst acoustic sensor 34′ may be positioned coaxially surrounding asecond acoustic sensor 36′, and the two output signals from each sensorare again delivered to a combiner 38′ to produce a single signalrepresenting the output of the sensor. It will be appreciated thatpiezoelectric sensors and other forms of acoustic sensors may havedifferent physical geometries to correspond to different frequencyresponse characteristics that may be desired for acoustic sensors. Thesedifferent physical geometries may permit adjacent positioning of sensorsas shown in FIG. 4A-1, or permit concentric positioning of sensors asshown in FIG. 4A-2.

[0036] Referring to FIG. 4B, it can be seen that through the use of acombiner such as 38 or 38′ the frequency response characteristic ofmultiple sensors may be combined to advantageously produce a frequencyresponse characteristic of a more broadband nature as may be needed forTACT imaging. As illustrated in FIG. 4B, a first frequency responsecharacteristic 40 having a relatively lower frequency band of responseis combined with a second frequency response characteristic 42 having arelatively higher band response to produce a combined response 44 havingbroader bandwidth than either of the response curves from which it iscreated.

[0037] Referring now to FIG. 5, the use of detector array 24 in TACTimaging as well as ultrasonic imaging can be explained. A furtheradvantage of detector array 24 is that it provides an array of adjacentacoustic sensors that may also be used in a conventional ultrasoundprocess for ultrasound imaging of the breast tissue in conjunction withor in addition to thermoacoustic imaging. Specifically, the acousticsensors in detector array 24 are coupled to a TACT receiver 46 and to aTACT processing system 48 for producing TACT images using acousticsignals detected by the detector array 24. Details of this process aredescribed in the above-referenced U.S. Pat. No. 5,713,356, and are notrepeated here. The resulting TACT-generated image may be displayed on adisplay for diagnostic purposes. Simultaneously, or as a separateimaging modality, the sensors on array 24 may be used for conventionalultrasound imaging of the subject tissue. Specifically, for thisapplication, an ultrasound beam steering delay circuit 54 is controlledby an ultrasound imaging system 52 to produce a narrow sweeping beam ofultrasound directed from the piezoelectric elements of detector array 24into the tissue of the breast. Echoes produced within the breast arethen received by the acoustic sensors in detector array 24 and deliveredto an ultrasound receiver 56, and then relayed to the ultrasound imagingsystem 52 using conventional ultrasound imaging techniques. As a result,an ultrasound image of the tissue may be created and presented ondisplay 50 overlaid with or as a substitute for comparison to theTACT-generated image produced by the TACT system 48. Combined ultrasoundand TACT imaging created in this manner may serve diagnostic purposesthat cannot be realized by either modality alone, by permittingdiscrimination of tissue structures that are more readily recognizedwith each modality, and permitting direct comparison of images producedby each modality by a clinician operating the scanning system andviewing display 50.

[0038] Referring now to FIGS. 6A-6E-2, the use of the waveguides 18 increating polarized electromagnetic radiation within the breast can beexplained. The polarization of electromagnetic radiation irradiating thebreast may affect the image produced. Specifically, polarization refersto the axes of oscillation of magnetic and electric field inelectromagnetic radiation, and therefore relates to the direction inwhich tissue is stimulated by electromagnetic radiation to producethermoacoustic effects. Different polarization directions may,therefore, produce different thermoacoustic reactions within tissue.Waveguide structures 18 can be manipulated to change the polarization ofradiation in the breast, and such changes may be useful in manipulatingthe generated image to produce enhanced images of structures ofinterest. For example, tissue structures that are elongated may bebetter imaged by polarization that is either aligned with orperpendicular to the elongated dimension of those structures.

[0039] FIGS. 6A-6E-2 are plan views of the thermoacoustic computedtomography scanning apparatus illustrated in FIG. 1. The interior endsof each of the waveguides 18-1-18-8 are shown schematically, as is thebreast tissue 22 being imaged. As seen in FIG. 6A, polarization directedvertically downward (into the paper as shown in FIG. 6A) can begenerated by orienting the polarization of radiation emitted from eachof the waveguides 18 to be also vertically downward (also into the paperas shown in FIG. 6A).

[0040] As seen in FIG. 6B, polarization that is horizontal (across thepaper as shown in FIG. 6A) can be created by producing horizontalpolarization at each of the waveguides 18 in the directions illustratedadjacent to each waveguide.

[0041] Referring to FIG. 6C, horizontal circular polarization 64 may beproduced in the tissue by appropriate polarization and phase delays tothe waveguides 18. Specifically, 90° phase delays are applied toradiation emitted by waveguides 18-1, 18-4, 18-5 and 18-8. Horizontalpolarization is provided by each of the waveguides as shown in FIG. 6Cin the direction shown in FIG. 6C.

[0042] As seen in FIG. 6D, vertical circular polarization may beachieved through alternative phase delay and polarization arrangementsshown in FIG. 6D. Here again, 90° phase delays are applied to radiationemitted from waveguides 18-1, 18-4, 18-5 and 18-8. Horizontal andvertical polarization is produced by the waveguides in the directionsillustrated in FIG. 6D.

[0043] Referring to FIG. 6E-1, vertically precessing electrical fieldmay be generated in the breast tissue 22 by an appropriate arrangementof polarizations and phase delays as shown in FIG. 6E-1. Specifically,90° phase delays are applied to the electromagnetic radiation emittedfrom waveguides 18-4 and 18-8, and alternating horizontal and verticalpolarization are applied to the waveguides as illustrated in FIG. 6E-1.The resulting electric field has a vertical component 68 which precessesabout a circular path 70 as illustrated in FIG. 6E-1. FIG. 6E-2 providesa side view of breast tissue 22 exposed to vertical precessing electricfield, showing that the direction of the electric field 68 is at anangle of 35° 72 from vertical and precesses about circular path 70 atthis angle.

[0044]FIG. 7 illustrates a handheld TACT scanner. This scanner is placedin physical contact with the surface 80 of the skin of a patient toimage tissue structures directly beneath the skin, such as a tumor orsuspicious mass 90. A membrane 82 on the outer surface of the scanner ispressed against the skin surface 80 to achieve good acoustic couplingthereto. The scanner utilizes a waveguide 84 for launchingelectromagnetic radiation into the tissue to stimulate a thermoacousticresponse. Resulting thermoacoustic signals are received by a pluralityof acoustic sensors 86-1 through 86-8 arranged about the periphery ofthe handheld scanner. An acoustic coupling media such as a water solublegel is contained between membrane 82 and sensors 86-1 through 86-8 toachieve good acoustic coupling from the tissue to the sensors. In use,thermoacoustic responses produced in the tissue and received by sensors86-1 through 86-8 are back projected in the manner described in theabove-referenced U.S. Pat. No. 5,713,356, to form an image of the tissuestructures beneath the skin such as the mass 90.

[0045] While the present invention has been illustrated by a descriptionof various embodiments and while these embodiments have been describedin considerable detail, it is not the intention of the applicants torestrict or in any way limit the scope of the appended claims to suchdetail. Additional advantages and modifications will readily appear tothose skilled in the art. The invention in its broader aspects istherefore not limited to the specific details, representative apparatusand method, and illustrative example shown and described. Accordingly,departures may be made from such details without departing from thespirit or scope of applicant's general inventive concept.

What is claimed is:
 1. A thermoacoustic imaging system for imagingstructures of tissue, comprising a source of electromagnetic radiationfor irradiating said tissue to stimulate a thermoacoustic response, anacoustic coupling media for acoustically coupling said tissue to anacoustic sensor array, and an acoustic sensor array for detecting saidthermoacoustic response, comprising an array of sensors arranged on asurface, said surface being rotatable about an axis to position saidsensors in a plurality of positions for detecting said thernoacousticresponse, the angular extent of said surface about said axis subtendingan angle that is less than a full revolution.
 2. The thermoacousticimaging system of claim 1 wherein said surface comprises a plurality offlat sections arranged to form an arc extending from a side nearer tosaid surface to a side further from said surface.
 3. The thermoacousticimaging system of claim 2 wherein said flat sections are arranged suchthat a geometric center of each section is equidistant from a commonpoint.
 4. The thermoacoustic imaging system of claim 3 wherein saidcommon point is on said axis.
 5. The thermoacoustic imaging system ofclaim 3 wherein said flat sections each carries a plurality of acousticsensors.
 6. The thermoacoustic imaging system of claim 5 wherein saidsection nearer to said axis carries fewer acoustic sensors than saidsections further from said axis.
 7. The thermoacoustic imaging system ofclaim 1 wherein one or more of said acoustic sensors comprise dualpiezoelectric sensors, and a combiner for combining signals from saiddual piezoelectric sensors to form a combined signal.
 8. Thethermoacoustic imaging system of claim 7 wherein said dual piezoelectricsensors comprise a smaller sensor and a larger sensor surrounding saidsmaller sensor.
 9. The thermoacoustic imaging system of claim 1 furthercomprising a thermoacoustic computed tomography receiver coupled to saidsensors for combining thermoacoustically generated acoustic signalsreceived from said tissue by said sensors to form an image of saidtissue.
 10. The thermoacoustic imaging system of claim 9 furthercomprising an ultrasound beam steering circuit coupled to said acousticsensors for stimulating said sensors to produce an ultrasound beamdirected into said tissue, and an ultrasound receiver for receivingultrasound echoes received from said tissue by said sensors.
 11. Thethermoacoustic imaging system of claim 10 further comprising anultrasound imaging system coupled to said ultrasound receiver forcombining ultrasound signals to form an image of said tissue.
 12. Athermoacoustic imaging system for imaging structures of tissue,comprising an acoustic coupling media for acoustically coupling saidtissue to an acoustic sensor array, an acoustic sensor array fordetecting said thermoacoustic response, comprising an array of sensorsarranged on a surface, and a source of electromagnetic radiation forirradiating said tissue to stimulate a thermoacoustic response,comprising a plurality of sources arranged about said tissue andproducing synchronized electromagnetic radiation in varyingpolarizations or phases to irradiate said tissue with electromagneticradiation of a desired polarization.
 13. The thermoacoustic imagingsystem of claim 12 wherein said plurality of sources comprise awaveguide positioned to launch electromagnetic radiation toward saidtissue.
 14. The thermoacoustic imaging system of claim 12 wherein saidplurality of sources comprise a plurality of waveguides positioned tolaunch electromagnetic radiation toward said tissue, said waveguideslaunching electromagnetic radiation in varying polarizations or phases.15. The thermoacoustic imaging system of claim 14 wherein said pluralityof sources launch electromagnetic radiation toward said tissue withvertical and horizontal polarization.
 16. The thermoacoustic imagingsystem of claim 14 wherein said plurality of sources launchelectromagnetic radiation toward said tissue with relative phase shiftsof zero and ninety degrees.
 17. A handheld thermoacoustic imaging systemfor imaging structures of tissue, comprising a device sized to be heldwithin the human hand, the device comprising a source of electromagneticradiation for irradiating said tissue to stimulate a thermoacousticresponse, and an acoustic sensor array for detecting said thermoacousticresponse, comprising an array of sensors arranged on a surface.